Historically, oil wells which must be produced by artificial lift have used a horsehead-type pumping unit such as those made by Lufkin Industries and others. To counterbalance the weight of the sucker rod string, counterweights are used, either mounted on the walking beam or a rotary-type mounted on the gear box Pittman arm. Another class of pumping unit (also made by Lufkin) uses an air cylinder in place of the metal counterweights. The effect is roughly the same.
The present invention has several advantages, one being omission of a gear box. A combined hydraulic and pneumatic system powers the walking beam and simultaneously counterbalances the weight of the sucker rod string. The pneumatic pressure acts directly on an exposed piston rod or indirectly on a second and larger piston as shown in alternate forms. The hydraulic cylinder is powered by a hydrostatic hydraulic pump which is, in turn, driven by a suitable power source such as an electric motor. To reciprocate the pump up and down in a sine wave motion, a control signal is formed either mechanically or electronically, and the motion of the cylinder is fed back to the input where the signals are compared. In other words, a closed loop feedback system drives the hydraulic cylinder. All forms or shapes of waive motion are feasible, enabling better downhole recovery.
The second and most important aspect of the present invention is the ability to control all factors of the hydraulic cylinder's motion (i.e., speed, dwell time and waveform). Even though a constant prime mover powers the system, one obtains the added ability to control and conform pump motion to the actual production requirements of the oil well. One signal indicating actual well flow is available from several sources such as the production flow from the well being produced across an orifice opening and the pressure drop across the orifice converted into a signal to vary pumping. A more advanced situation involves processing data generated by the pumping unit (position of rods, production pressure, etc.) to control pumping via analysis as taught by Gibbs in U.S. Pat. No. 3,343,409. Gibbs enables analysis of conditions at the bottom of the well and a manner of using this analysis of data to signal the pumping unit servodrive to change the driving signal to achieve maximum production. Gibbs has a rather good textbook discussion of the advantage of obtaining well production just at the pumped off point, and with this invention, it is possible to do that on a continuous basis without attention by the lease operator.
An important aspect of this invention is the linking of the operation of the pumping unit to the actual production of the well. While this is best accomplished by the hydrostatic hydraulic unit proposed, it is not limited to this type prime mover. By using this or other variable speed devices, a retrofit to existing oil wells or artificial lift can be obtained.
Gibbs discusses the relationship between a tophole dynamic measurement and actual downhole pump requirements. The reference discloses that a long string of sucker rods distorts the force required and dynamic loading actually experienced at the pump. Through the teachings of Gibbs, it is possible to measure surface dynamic data and obtain better performance downhole. This apparatus enables the power plant to drive the walking beam in a controlled manner so that the movement of the walking beam is controlled in frequency, dwell time, wave shape and excursion.
The present invention is an improvement over the Lufkin equipment. The present invention is a structure which utilizes not merely a passive air tank for counterbalancing, but a dynamic combined hydraulic and pneumatic system. From the exterior, it can be seen to include a large air tank with protruding piston rod which functions to counterbalance the load of sucker rods hanging on the walking beam. The equipment, however, goes much further. Through the use of a hydrostatic hydraulic pump and a closed hydraulic circuit, it incorporates a double-rod, double-acting piston which is positively driven in both directions, the piston enclosed in a cylinder and there being a protruding piston rod whereby the hydraulic equipment strokes the walking beam to obtain the necessary pumping action. This more readily accommodates variations in operation. Variations include waveform, frequency, dwell time and excursion. Frequency, length of stroke, dwell time and waveform are important factors in controlling the pumping operation. This is an optimum range of conditions for a given producing well. It cannot be pumped off too rapidly, and yet, maximum production is obtained by pumping at an optimum high frequency waveform. If the well is pumped off, damage may occur in that the sucker rods may be bent by slapping the pumping element against the accumulated oil in the well if it is below level. Further, the length of stroke of the equipment is also very important in obtaining optimum production from the well. In general, long strokes and slow speed are best, the present invention lending itself to long stroke-slow speed operation.
The present invention is thus a pumping unit which can be adjusted quickly and easily to accommodate great variety in pumping motion. It also accommodates variations in counterbalance load. These variations are implemented by simply adjusting pressure regulators or valves, or sensing dynamic operation.
Alternate forms of the present apparatus are disclosed. One variation is the incorporation of an alternate form of a lubricating system in conjunction with the double piston arrangement. Another alternate form discloses two modes of connecting the pickoff apparatus which determines the position of the walking beam. This, in turn, is connected to the pump which controls delivery of the hydraulic oil under pressure for operation of the walking beam.
The control system can be modified to sense downhole load to thereby further modify the cycle of operation of the equipment.